Measurement of full-field (two-dimensional), time-resolved structural vibration, can be accomplished using laser Doppler vibrometry (LDV) or high-speed electronic speckle pattern interferometry (ESPI), with full-field data compiled from two-dimensional spatial scanning (in the case of LDV) or temporal gating (in the case of ESPI) over multiple independent measurements. When correctly synchronized, the time history of a typical single surface motion can be synthesized or reconstructed from the multiple independent measurements. These approaches, however, assume that the measurement conditions remain invariant while multiple and, supposedly, identical, sequential measurements are performed, making this approach undesirably slow. These approaches are thus suited to steady-state or well characterized vibrations, generated under benign or controlled conditions.
These approaches are also critically dependent on the ability to acquire a reliable timing or reference signal from a point on the moving surface while the sequential measurements are in process in order to correctly synchronize and reconstruct the spatio-temporal phase of the surface motion. This is often implemented by attaching a load cell to the surface or by employing a secondary stationary point vibrometer.
Most engineering environments do not satisfy these requirements. In routine tests, natural or forced excitation can give rise to steady state and/or non-steady state vibrations which can include transients, e.g. from impact, or coupled vibrations, generated under test conditions which may be difficult, or impractical, to reproduce consistently or repeatedly.
Reports in the scientific and patent literature cite the development of linear array multiple-beam laser vibrometers aimed primarily at reducing measurement times with respect to single beam scanning LDV. Existing instruments include linear arrays with up to 16 independent channels, which, together with opto-mechanical rotation or linear translation, can yield full-field data with concomitantly fewer sequential measurements.
Examples of such contemporary multiple-beam laser vibrometers are disclosed in United Kingdom patent GB 2372097A, entitled “MULTIPLE BEAM INTERFEROMETER”, issued in 2002, and in U.S. Pat. No. 7,116,426, issued on Oct. 3, 2006, entitled “MULTI-BEAM HETERODYNE LASER DOPPLER VIBROMETER”.
Both homodyne and heterodyne LDV systems based on single-point and linear array measurement techniques, as discussed above, have been extensively investigated. Such devices form the basis of various conventional commercial instruments. However, such devices do not provide adequate two-dimensional information for many practical applications because of the inherent data latency associated with opto-mechanical scanning of the beam(s) over the measurement surface. This approach is not capable of real-time vibration imaging, but is capable of synthesizing vibration images for a restrictive subset of well behaved surface vibrations.
Full-field or imaging vibrometry is also claimed using the method of high-speed electronic speckle pattern interferometry. Examples of contemporary high-speed electronic speckle pattern vibrometers are disclosed in U.S. Pat. No. 7,193,720, entitled “OPTICAL VIBRATION IMAGER” issued on Mar. 20, 2007. However, as is well known, the temporal bandwidth of the proposed (commercial) camera detector arrays are insufficient to support real time vibration imaging. Future developments are conjectured to address this limitation. Assuming such developments do follow, the adoption of CCD or CMOS pixel camera technology in full-field heterodyne vibrometry must still address the noise associated with DC detector bias, which become progressively more severe with increasing bandwidth.